Conventionally, a bicycle derailleur, as disclosed in, for example, Japanese Patent Publication Gazette No. Sho. 55-10,428, switches the driving chain exactly and quickly to the sprocket regardless of a difference between the number of teeth of the respective sprockets of the multistage sprocket assembly.
This conventional derailleur, as shown in FIG. 13, is so constructed that a movable member M is supported movably to a base member B through a pair of parallelogrammic link members L, and a guide pulley P.sub.1 is supported swingably to the movable member M around a horizontal support shaft A.sub.1 through a first support G.sub.1. The first support G.sub.1 is subjected to an elastic force acting clockwise through a first spring S.sub.1, a tension pulley P.sub.2 is supported rotatably to a second support G.sub.2 swingable around a shaft A.sub.2 for the guide pulley P.sub.1, and the second support G.sub.2 is subjected to an elastic force acting clockwise through a second spring S.sub.2.
The conventional example, however, uses the first and second springs S.sub.1 and S.sub.2 so that they need be balanced with accuracy, in order to carry out the most ideal chain switching while considering the difference between sprocket teeth numbers. Also, the springs S.sub.1 and S.sub.2, even when balanced with accuracy, cause variation in the elastic force due to machining and by fatigue after use. As a result, a problem has been created in that an interval between the guide pulley and the respective sprockets cannot be kept exactly constant.
Moreover, the use of first and second springs S.sub.1 and S.sub.2 increases the number of parts, deteriorates the workability, and leads to a high manufacturing cost as a whole.
In other words, in a case where the conventional derailleur switches the driving chain from a smaller diameter sprocket to a larger diameter one, the movable members M move in a parallelogrammic manner with respect to the base member B to push the chain carried by the guide pulley P.sub.1 against the side surface of the larger diameter sprocket, the chain is caught and lifted by the teeth to the larger diameter sprocket following rotation thereof, and then the springs S.sub.1 and S.sub.2 act on the supports G.sub.1 and G.sub.2, for the first time after the chain is lifted, to be switched to the same, whereby the supports G.sub.1 and G.sub.2 swing to shift the guide pulley P.sub.1. Accordingly, in a case where the teeth difference between the numbers of respective sprockets is greater, the chain, even when pushed against the side surface of a larger diameter sprocket, is not caught by the teeth if positioned at the sprocket body. Therefore the chain is neither lifted nor switched to the larger diameter sprocket.